There has been known one type of four-stroke engine comprising a crank chamber, and an oil pan disposed below the crank chamber to contain lubrication oil therein, wherein engine components, such as a crankshaft and a connecting rod contained in the crank chamber are lubricated by oil mist generated in the oil pan, as disclosed, for example, in Japanese Patent Laid-Open Publication No. 2001-207817. This four-stroke engine is an air-cooled type in which an airstream generated by a fan rotor flows between cooling fins formed on the outer surface of a cylinder block.
Generally, if the oil pan has a temperature higher than an acceptable temperature, lubrication oil contained in the oil pan will be prematurely deteriorated or vaporized. Thus, it is important to prevent the oil pan from being heated at an excessively high temperature so as to maintain the lubrication oil at an adequate temperature. Further, a high temperature in an engine is likely to cause seizure in bearing. Therefore, it is also important to prevent the four-stroke engine from being excessively heated up so as to provide enhanced durability of components of the four-stroke engine. If the excessive heating is avoided, the materials of the four-stroke engine can also be selected with enhanced flexibility.